The company's U-turn followed the judge's decision last week to throw out a corporate manslaughter charge following legal arguments.

As the defence was due to open at the Old Bailey, Balfour Beatty's lawyer Ronald Thwaites, QC, said the firm wished to plead guilty to a Health and Safety Act charge, the only one it still faced.

In the basis for its guilty plea, the firm - contracted to maintain track - stated: "There was no one single cause of the derailment, but an aggregation of causes."

It denied it had failed to comply with Railtrack's standards and said if re-railing had taken place, as planned, the track would not have broken causing the 115mph derailment which killed four people.

It added: "Re-railing had been scheduled as a priority. Parties other than the company were wholly responsible for the carrying out of scheduled re-railing and for the failure to re-rail by the date of the derailment."

When he opened the case, Richard Lissack, QC, prosecuting said the derailment was the result of a "cavalier approach to the safety of those in trains".

Mr Justice Mackay instructed the jury to find the firm guilty. Sentencing will come later and there is no limit to the fine it may face.

Five rail bosses were also cleared last week of manslaughter due to gross negligence. They are: Alistair Cook, 52, Sean Fugill, 52, and Keith Lea, 55 - all senior management of Railtrack, now called Network Rail - and Anthony Walker, 48, former regional director of Balfour Beatty and Nicholas Jeffries, 50, a civil engineer for Balfour Beatty. They are still accused of breaching safety standards and the judge said the case against them would proceed in the normal way. "You should not be influenced either way by this change of plea," he told the jury.

Balfour Beatty's decision to plead guilty five months into the trial prompted a furious reaction from union leaders who accused the company of dragging out the case, so denying the victims' compensation.

Keith Norman, general secretary of train drivers' union Aslef, said: "I'm sickened and astonished. This terrible saga began with four tragic deaths in the Hatfield train crash.

"As if this were not enough, the aftermath has shown us the most cynical side of big business.

"Last week, individual managers got off scot free, although it had been established that they had ignored clear warnings that the track was unsafe.

"British industry has today assumed the moral standards of Al Capone. I feel sickened as a trade union leader. I can't bear to think how the bereaved must feel."